Pulse echo distance measurement

ABSTRACT

The invention concerns pulse echo distance measurement and in particular a method and apparatus for calculating such a distance by sensing multiple reflections of a given pulse signal. By sensing multiple reflections, and not just the primary echo, internal time delays which would otherwise cause systematic errors may be simply eliminated and the multiple readings obtained may be utilized to provide system self-diagnostic checks, eliminate spurious information and provide an accurate measure of distance. A particular advantage of the method is that very small distances between sensor and target may be measured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an improved method and apparatus for use inpulse echo distance measurement.

2. Description of the Related Art

Pulse echo methods are well known for use in distance measuring. In suchmethods a pulse of radiation is transmitted towards a target, reflectedat the target, and received by a sensor and the time elapsed betweentransmission and the time when the first or primary echo is receivedfrom the target is measured. Multiplying this travel time by the pulsepropagation velocity gives twice the distance to the target. Types ofradiation commonly used are electromagnetic radiation (radar), opticalradiation (lidar) or acoustics (sonar). The target may be a solid objectwhose position is to be pinpointed or the target may be the boundarybetween a liquid and a gas or between two or more liquids to measure thedepth of one or more of the liquids. The liquid may be flowing within achannel, for example, a river or may be non-flowing, in a natural basinor within a tank. Typically in such systems, the signal is an ultrasonicsignal using a submerged ultrasonic sensor or the sensor may be fixed toa rigid structure in air above a target surface, looking down on thetarget. In a further prior art arrangement, a fixed submerged sensor ismounted to a structure such as a bridge and utilized to monitor adistance from the sensor to the surface of, for example, a river bed.Variations in distance imply that either the bridge is shifting or thatthe bridge foundations are being eroded.

Such prior art devices determine the value relating to the primary echo.This is achieved by measuring the time elapsed between applying anelectronic stimulus to the transmitting element and the detection of thefirst reflected electronic echo signal from the receiving element. Thismeasurement contains delays not related to the pulse travel time throughthe liquid due to the delay between the stimulus to the transmittingelement and the actual transmission of a signal and the time travelledwithin the sensor until it reaches its surface before being directedtowards the target. Similarly there is a delay between the signalreaching the sensor surface and travelling to the receiving element andthen the detection of the echo signal from the receiving element. Thesedelays which may vary with operating conditions have to be removed bycalibration. This can lead to either expensive calibration or errors inthe system. The primary echo can be confused by spurious echoes frommaterials within the liquid, by missing echoes and when the primary echofalls within the blanking period (to be discussed later). This can causeunnecessary errors.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a methodfor measuring a distance between a sensor surface and a target, themethod comprising:

stimulating a pulse transmitter to transmit a pulse signal from a sensorsurface towards a target;

sensing an echo reflected from the target to the sensor and received bya pulse receiver to generate a first pulse received signal;

sensing at least a further echo reflected at least once from the sensorsurface to generate a second pulse received signal and determining thetime delay between the first and second pulse received signals to give avalue proportional to the time taken for the pulse to travel from thesensor surface to the target and thus the distance between the sensorsurface and the target.

Neither of these echoes need be the primary echo (i.e. the first echogenerated when the pulse travels from transmitter to target and back),but may in fact be second, third, fourth echoes etc. generated as aresult of secondary reflections between target and the sensor surface.

According to a second aspect of the invention, there is providedapparatus for measuring the distance between a sensor surface and atarget, the apparatus comprising:

a pulse transmitter arranged on receiving a stimulate transmissionsignal to transmit a pulse signal from the sensor surface towards thetarget;

a pulse receiver arranged to sense echoes of the pulse received at thesensor surface and generate pulse received signals; and,

control means coupled to each of the transmitter and the receiver,

wherein the apparatus is operable to stimulate the pulse transmitter totransmit a pulse signal, to receive a first pulse received signal fromthe receiver when an echo is received at the receiver and a second pulsereceived signal from the receiver when a further echo is received at thereceiver, and on the basis of the delay between the first and secondpulse received signals to generate a value proportional to the timetravelled between the sensor surface and the target, from which thedistance travelled can be calculated.

A primary echo signal is, as explained previously, the one generated bya signal which has travelled through the sensor body into thetransmission medium, been reflected at the target and then passed backthrough the medium and the sensor body to the receiving element, Otherecho signals occur because some of the pulse energy arriving back at thesensor is reflected back once more into the medium and undergoes asecond reflection at the target before returning to the sensor and soon, further echoes occurring as the ultrasound pulse makes multipletransits between the sensor and the target until the pulse energy isdissipated by attenuation and dispersion. In preferred embodiments aplurality of such further echoes are detected.

By using one or more of these further echoes, time delays due to delaysin pulse propagation caused by the delay between the stimulatetransmission signal being supplied to the transmitter element and thegeneration of the pulse, the delay due to the time taken for the wave topropagate through the sensors body and the delay between the receipt ofthe reflected pulse and the generation of the pulse received signal canall be eliminated. This can be seen in more detail if FIG. 3 of theaccompanying drawings is referred to, in which:

t_(t) is the time delay between applying a stimulate transmissionelectronic signal to the transmitter element and the generation of apulse

t_(p) is the time for the pulse to propagate through the sensor body

t_(x) is the time for the wave to travel from the sensor surface to thetarget and

t_(r) is the time delay between pulse arriving at the receiving elementand the generation of a pulse received electronic signal.

In algebraic form the arrival times for a primary echo, t₁, and echoest₂, t₃, t₄ . . . t_(n) are given below

    t.sub.1 =t.sub.t +t.sub.r +2·t.sub.p +2·t.sub.x( 1)

    t.sub.2 =t.sub.t +t.sub.r +2·t.sub.p +4·t.sub.x( 2)

    t.sub.3 =t.sub.t +t.sub.r +2·t.sub.p +6·t.sub.x( 3)

    t.sub.4 =t.sub.t +t.sub.r +2·t.sub.p +8·t.sub.x etc(4)

More generally the n_(th) echo occurs at time t_(n) given by

    t.sub.n =t.sub.t +t.sub.r +2·t.sub.p +2n·t.sub.x( 5)

From FIG. 3 and the above equations, it is clear that the echo sequenceis periodic and that the periodic interval (i.e the time intervalbetween the echoes at t_(n) and t_(n+1)) depends only on the travel timebetween the sensor surface and the target. By using the presentinvention, differences between the arrival times of the echo componentsare measured since each component is subject to the same unknown delayst_(t), t_(p), t_(r). The effect of the differencing process is to removethese unknown values. So that, for a Z^(th) echo and Y^(th) echo (Z>Y)the time 2t_(x) for a pulse to travel from sensor surface to target andback is given by: ##EQU1##

The invention encompasses a range of methods for determining theperiodic interval of the echo signal. This contrasts with traditionalpulse echo systems of the prior art in which only the time t₁ to receivethe primary echo signal is measured and all other echoes are ignored.

Traditional pulse echo systems make one time delay measurement for eachpulse transmitted. The method in accordance with the invention makes anumber of delay measurements per pulse. This means that the longer totaltime delay is measured and divided by the number of echo intervals whichresults in an effective improvement in resolution.

Traditional pulse echo systems can be confused by spurious echoes frommaterials within the liquid and by missing echoes when the primary echofalls within the blanking period (to be discussed hereinbelow). Theinvention gives access to several simultaneous measurements of the pulsetravel time which may be cross checked to validate the pulse echosignal. In simplest form, an echo signal giving n time measurements t₁to t_(n) provides n-1 first differences which are each utilisable togive an estimate of the pulse travel time. Checking that all n-1 valuesdiffer by less than some predetermined limit, for example, a fewmicroseconds gives a criterion by which the pulse echo signal may beaccepted or rejected. Pulses which give an estimate of travel timeclearly too short in comparison to other estimates are rejected as asystem fault or, for example, as being due to spurious reflections, suchas may be generated by extraneous material passing between sensor andtarget, and undue delays between received echoes giving clearly too longan estimate of travel time may indicate that a reflection or pulse is"missing". In such a manner, the apparatus and method may performpowerful self-diagnostic checks. Furthermore, once an echo has beenverified in the above manner the different estimates of travel time maybe averaged to give a more precise estimate of travel time and hencedistance between sensor surface and target.

Use of the method and apparatus in accordance with the present inventiongives a further advantage in that the system overcomes a problem whicharises with traditional pulse echo measurement systems. A typical pulseecho signal consists of a feed-through signal, a primary echo at time t₁and a sequence of secondary echoes of decreasing magnitude at furthertimes t₂, t₃, t₄ etc. The feed through signal occurs when there is adirect path between the elements which transmit and receive the signalwhich may often be physically the same component. This is sometimesremoved by the process of blanking, i.e. forcing the received signallevel to zero until a certain time t_(b) has elapsed after the pulsetransmission.

The blanking time means that there is a time following pulsetransmission in which echoes cannot be received. This limits the minimumdistance that can be measured by traditional systems which can onlyoperate when t₁ >t_(b). For submerged ultrasonic systems, this limitingvalue is typically 50-100 mm. However, use of the method in accordancewith the present invention avoids the problems caused by blanking sincefurther echoes arriving after the blanking time are processed. Theminimum distance that can be measured by particular apparatus is limitedonly by the ability of the receiver and controller to discriminatebetween the arrival of each echo. In practice, depth measurement down toless than 10 mm can be achieved.

The invention is applicable with particular advantage to systems formeasuring the depth of a liquid. The sensor may be arranged at thebottom of the body of liquid, for example on a river bed with thesensor's surface facing upwards towards the upper surface of the liquidwhich in this case is the target. Alternatively, the sensor may bemounted in the air above the liquid facing downwards towards the uppersurface of the liquid which is the target. Waves directed towards thesurface of the liquid will be reflected at the surface. Alternatively,the sensor could be mounted at or below the liquid surface (for examplefloating on the surface) and directed towards the base of a vessel orriver bed so that the base becomes the target.

The sensor may be arranged within a stabilizing tube extending from thesensor surface to the target so that the ultrasound or other wave pathis confined but the system also works with an unconfined path. The useof a stabilizing tube may damp movement of the surface of the liquid butproblems can occur when the liquid includes any solids which can becomeentrained in the perforations into the stabilizing tube. Preferably thestabilizing tube includes at its upper surface at least one aperturewhich can be used to control the flow rate of air from the stabilizingtube, and hence liquid into the stabilizing tube and this reduces theflow of solids into the stabilizing tube. Alternatively, flow rate canbe regulated by an aperture at the bottom surface of the stabilizingtube.

The control means which may be a microprocessor may be programmed tocalculate the volume of liquid within a vessel or alternatively may beprogrammed for use in connection with a weir, or flume, or similardevice to monitor the flow rate of liquid within a channel.

Preferably the sensor includes means for measuring the temperature ofthe liquid, and the microprocessor receives this signal to calculate thewaves (pulse) velocity within the liquid. This is because the speed oftravel of the wave within the liquid will be dependent on thetemperature. In this case, the apparatus may include display means fordisplaying the temperature.

The signals output by the microprocessor may be frequency, voltage,current, or serial or parallel digital outputs according to the use tobe made of them. The signals can be used to record, calculate or displaydepth and/or temperature measurements or any other parameter derivabletherefrom or to control other apparatus dependant on such parameters.For example, the signal may be fed to pumps for correcting the liquidlevel.

The sensor surface may be shaped to focus echoes and maximise the numberof echo signals which may be processed.

Preferably the microprocessor is programmed to process the echo delaysin one of a number of mathematical calculations to ensure the mostaccurate measurement of the distance. Such methods are described in moredetail in connection with the accompanying drawings. However, it will beapparent to the skilled addressee of the specification that there areother methods of processing the time delay signals to calculate thedistance.

DESCRIPTION OF THE DRAWINGS

A method and apparatus of pulse echo distance measurement will now bedescribed, by way of example only with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic view of a submerged sensor used for liquid depthmeasurement;

FIG. 2 is a graph of echo amplitude against time, for a typical pulseecho signal;

FIG. 3 is a schematic breakdown of the composition of a pulse signal;

FIG. 4 is an echo interval histogram of number of echoes againstinter-echo interval;

FIG. 5 is a graph of echo signal auto-correlation of number of echoesagainst time difference;

FIG. 6 is a graph of echo arrival time against echo number; and

FIG. 7 is a schematic section through a depth sensor in accordance withthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present apparatus is shown schematically in FIG. 1 and FIG. 7 and isone which operates using ultrasound pulses. The apparatus comprises asensor 1 having mounted within it a piezoelectric crystal 3 which isoperable to transmit and receive ultrasound pulses. Although FIG. 1appears to show two piezoelectric crystals 3, this is for aiding clarityof that illustration and, in practice, usually only one crystal 3 asshown in FIG. 7 is required and this acts as transmitter and receiver.The piezoelectric crystal is coupled via interface circuit 5 tomicroprocessor control 7. A pulse transmit signal is generated bymicroprocessor 7 via interface 5 to piezoelectric crystal 3 to radiate ashort pulse of ultrasound of a few micro seconds. Primary and secondaryecho signals are received by the crystal 3 and are fed via interfacesignal 5 to the microprocessor which is programmed to detect and recordthe arrival of each component of the echo sequence with a timingresolution of one micro second.

The sensor also includes a temperature sensor 9 coupled to themicroprocessor 7 to measure the liquid temperature from which itdetermines the ultrasound propagation velocity by means of a look uptable stored in memory (not shown).

Microprocessor 7 is coupled to a power input via cable 11 which also isused to transmit an output signal.

In use the sensor 1 is placed at the base of the body of the liquid. Themicroprocessor 7 stimulates the crystal to transmit an ultrasonic pulse13 from the sensor surface 15 towards target 17 which in this case isthe boundary between the liquid and the air above it. At the boundary apulse 19 is reflected towards the crystal 3.

A typical pulse echo signal is illustrated in FIG. 2. Given that thecrystal 3 acts as both a transmitter and receiver of the ultrasound, afeed-through signal 21 occurs at transmission. Primary echo signal 23 isgenerated by ultrasound which has travelled through the sensor body tosurface 15 into the liquid being reflected at the liquid surface andthen passed through the liquid and the probe body to the receivingelement. The secondary echo signals 25 occur because some of the pulseenergy arriving at the sensor surface 15 is reflected back into theliquid and undergoes a further reflection at the liquid surface 17before returning to the sensor 1. Further echoes at t₃, t₄ etc occur asthe ultrasound pulse makes multiple transits between the sensor and theliquid surface 17 until the pulse energy is dissipated by attenuationand dispersion. The composition of the ultrasound path associated withthe primary and secondary echo signals is illustrated schematically inFIG. 3 which has already been discussed in relation to equations (1) to(6). In terms of the different parameters t_(t), t_(p), t_(x) and t_(r)of those equations, these relate as follows to the specific embodimentof FIG. 7:

t_(t) is the time delay between the microprocessor 7 transmitting anelectronic signal to the transmitter element 3 and the generation ofultrasound by the crystal 3;

t_(p) is the time for the ultrasound to propagate through the sensorbody towards sensor surface 15;

t_(x) is the time for the ultrasound to travel from the sensor surface15 to the liquid surface 17 (and also for the time taken to travel fromthe liquid surface 17 back to the sensor surface 15); and,

t_(r) is the time delay between the ultrasound arriving at crystal 3 andthe generation of a signal received by microprocessor 7.

As mentioned previously, it is clear that the echo sequence is periodicand that the periodic interval, i.e. the time interval between theechoes t_(i) and t_(i) +1 depends only on the pulse travel time betweenthe sensor surface 15 and the liquid surface 17 and on none of the otherfactors affecting t_(t), t_(r) and t_(p). There are a number of ways ofprocessing the sequence of echo arrival times recorded by themicro-processor in order to determine the pulse travel time and toverify this measurement.

An echo signal yielding a sequence of n arrival time measurements (s₁ tos_(n)) provides n-1 first differences (i.e. s₂ -s₁, s₃ -s₂, etc.) whichare each estimates of the pulse travel time. Taking the mean of thesedifferences gives the pulse travel time from the sensor to the targetand back. This is numerically equal to calculating ##EQU2##

By testing all of the individual differences to ascertain that each iswithin a small interval of the mean value, typically a few microseconds,the reliability of the measurement may be confirmed.

Alternatively, the n-1 first differences may be plotted to form ahistogram as in FIG. 4. This histogram may contain data from a singlepulse echo signal or may be the accumulation of data from several pulseechoes. The position of the peak of this histogram is the required pulsetravel time. This peak position may be obtained by numeric techniques orby analytical means. The height of the histogram peak above any baselinenoise gives an indication of the reliability of the pulse travel timemeasurement.

Alternatively, the echo interval histogram of FIG. 4 may be extended toinclude higher differences (i.e. s_(i+2) -s_(i), s_(i+3) -s_(i) etc).The resulting histogram is the positive half of the symmetricalauto-correlation function. This function has a comb like structure asshown in FIG. 4 from which the pulse travel time may be determinedeither directly or by techniques such as Fourier analysis.

However, the most effective method of calculating the pulse travel timefrom the sequence of echo arrival times s₁, s₂, . . . s_(n) is by linearregression. Plotting each echo arrival time s_(i) against i gives in theideal case a set of points on a straight line of slope 2·t_(x) as shownin FIG. 6. If the first echo arrival time s₁ corresponds to the primaryecho it is easy to show that the best straight line fit (least squareerror) to the data has a slope given by ##EQU3## and the intercept withthe echo arrival time axis occurs at ##EQU4## The microprocessor isprogrammed to use this method to calculate the slope value which is thebest estimate of the time interval between successive echoes--the pulsetravel time. The position of the intercept with the echo arrival timeaxis which should occur at t_(t) +t_(r) +2·t_(p) is used for echo signalverification. When this intercept occurs at an implausible value, i.e.less than zero or greater than a few microseconds, the signal isdiscarded. Otherwise the calculated slope is taken as a measure of thepulse travel time. Alternatively, the mean square deviation of thepoints from the best fit straight line can be used as the parameter bywhich the sequence is accepted or rejected.

In the case where it cannot be guaranteed that the first echo arrivaltime s₁ corresponds to the primary echo it is possible to modify theecho arrival time sequence by subtracting s₁ from each element of thesequence and removing the first (zero) item (i.e. s₂ -s₁ becomes s₁, s₃-s₁ becomes s₂, s₄ -s₁ becomes s₃, etc.). The micro-processor then usesthe above procedure to calculate the best estimates of the slope and theintercept, which should now occur at an echo arrival time of zero. Themeasurement is accepted only if the calculated intercept is within a fewmicroseconds of zero. Otherwise the sequence is discarded.

Multiplying the pulse travel time by the ultrasound velocity gives thedepth measurement. Several such measurements are averaged to produce thefinal depth measurement which is output by the microprocessor as afrequency signal suitable for input to a data logger.

Experimental data showed that depth measurements from 10 mm to 5 metresare possible with systematic errors within 0.2% of reading. Thissuperior to the performance of pressure sensors which are at presentwidely used. The ultrasonic system is significantly better than pressuresensors at low depths and has a comparable performance to systems usingfloat, counterweight and shaft encoder.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

I claim:
 1. A method for measuring a distance between a sensor surfaceand a target, the method comprising the steps of:stimulating a pulsetransmitter to transmit a pulse signal from the sensor surface towardsthe target; and sensing a plurality of echoes, said plurality of echoescomprising a) an echo reflected from the target to the sensor andreceived by a pulse receiver to generate a pulse received signal at acorresponding echo arrival time and b) an echo reflected at least oncefrom the sensor surface to generate a further pulse received signal witha respective arrival time; and determining the time delay between thepulse received signals to therein give a value proportional to the timetaken for the pulse to travel from the sensor surface to the target and,a distance between the sensor surface and the target, wherein an arrivaltime, t_(n) of an echo corresponding to that of a transmitted pulsewhich is being reflected n times by the target is given by the followingequation:

    t.sub.n =t.sub.t +t.sub.r +2·t.sub.p +2n·t.sub.x

where, t_(t) represents a time delay between applying a stimulatetransmission electronic signal to a transmitter and the generation of apulse, t_(p) represents the time taken for the pulse to propagatethrough a sensor body, t_(x) the time taken for the pulse to travel froma sensor surface to the target and t_(r) represents the time delaybetween a pulse arriving at the receiving element and the generation ofa pulse received signal.
 2. A method according to claim 1, wherein aplurality of further echoes are detected.
 3. A method according to claim2, wherein arrival times of the plurality of further echoes aremeasured.
 4. A method according to claim 1, wherein the time, 2t_(x)taken for a pulse to travel from the sensor surface to the target andback based on a zth echo and yth echo is given by: ##EQU5##
 5. A methodaccording to claim 1, wherein a plurality of arrival times are measuredand the pulses travel time are calculated from the sequence of measuredecho arrival times by linear regression.
 6. A method according to claim1, wherein a plurality of time delays between pulse received signals aredetermined and the multiple values calculated for the travel timecompared and, wherein, a result of the comparisons is used to carry outself-diagnostic checking.
 7. The method according to claim 5, whereinsaid pulse travel time 2t_(x) is given by a slope of a line obtainedduring said linear regressing process and an intercept of said line withthe echo arrival time access is used to provide a self-diagnosticfunction wherein when said intercept corresponds to an implausiblevalue, the calculated pulse travel time to t_(x) is rejected. 8.Apparatus for measuring the distance between a sensor surface and atarget, the apparatus comprising:a pulse transmitter arranged onreceiving a stimulate transmission signal to transmit a pulse signalfrom the sensor surface towards the target; a pulse receiver arranged tosense echoes of the pulse received at the sensor surface and generatepulse received signals; and control means coupled to each of thetransmitter and the receiver, wherein said pulse receiver senses aplurality of echoes, said plurality of echoes comprising an echoreflected from the target to the sensor and received by said pulsereceiver to generate a pulse received signal at a corresponding echoarrival time and an echo reflected at least once from the sensor surfaceto generate a further pulse received signal with a respective arrivaltime and said control means for determining the time delay between thepulse received signals to give a value proportional to the time takenfor the pulse to travel from the sensor service to the target and, adistance between the sensor surface and the target, wherein an arrivaltime, t_(n) of an echo corresponding to that of a transmitted pulsewhich is being reflected n times by the target is given by the followingequation:

    t.sub.n =t.sub.t +t.sub.r +2·t.sub.p +2n·t.sub.x

where, t_(t) represents a time delay between applying a stimulatetransmission electronic signal to the pulse transmitter and thegeneration of a pulse, t_(p) represents the time taken for the pulse topropagate through a sensor body, t_(x) the time taken for the pulse totravel from a sensor surface to the target and t_(r) represents the timedelay between a pulse arriving at the receiving element and thegeneration of a pulse received signal.
 9. Apparatus according to claim8, wherein the pulse transmitter and the pulse receiver form part of asingle component.
 10. Apparatus according to claim 8, wherein the sensoris provided with a stabilizing tube extending from the sensor surface tothe target so that a path of the pulse is confined, said stabilizingtube including at least one aperture which can be used to control theflow rate of air or liquid from the stabilizing tube.